Fix colormap handling for containers. (#2450)

Fix colormap handling for containers. (#2450)

This commit correctly handles colormaps by

* Using the static default colormap we determine on startup if the
  con has the corresponding depth. This avoids creating pointless
  colormaps.
* Not freeing the default colormap to not have stray colormaps on
  containers. This fixes an issue with certain programs such as xwd.
* Creating a custom colormap when necessary and freeing it when the
  container is killed.

fixes #2435